1. Field of the Invention
This invention relates to the field of nitrogen oxide emission control, and, in particular, it relates to methods for chemically reducing nitrogen oxides and thereby reducing the emission of such oxides to the atmosphere or otherwise.
2. Description of the Art
Several nitrogen oxides are employed in and/or are produced by a variety of processes and are known to constitute undesirable emissions to the work place and environment. Nitrogen oxides which are employed in or are produced by chemical processes include nitrous oxide (N.sub.2 O), nitric oxide (NO), nitrogen trioxide (N.sub.2 O.sub.3), nitrogen tetraoxide [NO.sub.2 or (NO.sub.2).sub.2 ], and nitrogen pentaoxide (N.sub.2 O.sub.5). Numerous processes emit one or more of such nitrogen oxides. Illustrative of such processes are relatively high temperature fuel combustion systems, e.g. systems which operate at flame temperatures of 870.degree. C. or more, and a variety of chemical processes including the manufacture and concentration of nitric acid; processes involving the nitration of organic compounds in either liquid or vapor phase by reaction of the organic compounds with nitric oxide or nitric acid; nitric acid or nitric oxide containing metal-treating systems widely employed for pickling, etching and descaling ferrous metal articles such as wire and metal plate; and processes for the recovery of metals such as copper, molybdenum, gold, platinum, palladium, and other metals or compounds from their respective alloys or ore concentrates. Precious metals such as gold and platinum are frequently recovered from their alloys by treatment with aqua regia, and copper and molybdenum compounds such as the sulfides and sulfates are frequently recovered from ore concentrates by treatment with concentrated nitric acid. Electric discharge processes such as electric discharge metal machining and welding and certain high intensity light sources such as carbon electrode lights are also known to emit nitrogen oxides, although such processes are not generally considered to be major contributors of nitrogen oxides to the environment. The principal contributors to nitrogen oxide emissions are nitric acid manufacture and concentration systems, metal-treating systems, and fuel combustion systems such as power plants, industrial fuel and waste burners and automotive vehicles.
The U.S. government has established standards for maximum emissions of nitrogen oxides of all types from the major nitrogen oxide sources, and several state governments have enacted even more stringent regulations.
Presently, nitrogen oxide emissions can be controlled by several processes including absorption, adsorption, catalytic reduction, and selective reduction, and by modifying fuel burners including industrial and automotive burners and engines. Adsorption systems are usually employed to remove nitrogen oxides from vapor streams by contact with molecular sieves. These systems are relatively expensive to install, regenerate, and maintain, and they have relatively limited capacities. Absorption usually involves contact with water and/or an aqueous base such as ammonia, sodium hydroxide and the like.
It is also known that nitrogen oxides can be absorbed in and chemically reduced by aqueous urea solutions. The urea can react with nitrogen oxides absorbed in the solution to chemically reduce them to elemental nitrogen and water. However, such systems have low absorption efficiencies and reaction rates.
The addition of acids to aqueous urea solutions to improve the ability of the solutions to remove nitrogen oxides from gas streams has also been suggested. One such process, disclosed by Warshaw in U.S. Pat. No. 3,565,575, involves the use of solutions which contain about 1 to about 30 grams of dissolved urea per 100 ml of solution together with dissolved free acid in a proportion up to about 10 percent by volume. The use of urea to remove nitrogen oxides from flue gas and to prevent nitrogen oxide emissions from catalyst regeneration at elevated temperatures above the boiling point of water has also been suggested. For instance, Arand et al., U.S. Pat. No. 4,325,924, disclosed that urea can be employed to remove nitrogen oxides from fuel-rich, reducing, flue gas streams at temperatures in excess of 1900.degree. F. while Goldstein et al., U.S. Pat. No. 4,061,597, disclosed that the level of nitrogen oxide emissions from heat treated catalysts can be reduced by conducting such catalyst treatment in the presence of urea at temperatures of about 300.degree. C. and higher.
Catalytic reduction is presently the principal method of controlling nitrogen oxide emissions from industrial fuel burning plants and generally involves the reaction of nitrogen oxides in the exhaust stream with excess fuel in the presence of a catalyst containing a precious metal such as palladium, platinum and/or rhodium. Although catalytic reduction can reduce nitrogen oxide emissions to acceptable levels, it does require certain process modifications such as the use of excess fuel and the consumption of all oxygen in the exhaust stream prior to contacting the catalyst. Such processes have the further disadvantage that they require relatively expensive catalysts which deactivate and are difficult to regenerate. Catalytic reduction also requires relatively high conversion temperatures and thereby results in relatively high exhaust system temperatures.
Selective reduction involves the reaction of nitrogen oxides with ammonia in the presence of a catalyst such as a base metal oxide and is sometimes used for the control of nitrogen oxide emissions from industrial gas-fired equipment. Suitable catalysts include the oxides of calcium, magnesium, platinum, palladium and/or rhodium. Like several other processes, selective reduction requires that oxygen or other oxidants be eliminated from the exhaust stream prior to catalytic treatment and requires close temperature control in the conversion zone. Furthermore, selective reduction is relatively inefficient unless the more expensive metal catalysts such as platinum, palladium and rhodium are employed, and even these catalysts gradually deactivate and are difficult to regenerate.
Automotive nitrogen oxide emissions can be reduced by catalytic reduction using relatively expensive platinum, palladium and/or rhodium catalysts on a solid support such as alumina. These systems suffer from the disadvantages associated with high exhaust temperatures and the necessity of eliminating oxygen from the exhaust stream prior to catalytic treatment. Other procedures for reducing nitrogen oxide emissions in automotive exhausts include reducing or eliminating spark advance, at least under some engine operating conditions, reducing air-fuel ratio, exhaust gas recycle, and the like, all of which significantly diminish engine performance and efficiency.
Most of the processes referred to above require the use of relatively large installations to achieve adequate efficiency and nitrogen oxide emission levels and often cannot be economically justified, particularly for small industrial operations. Also, many existing nitrogen oxide emitting facilities cannot be easily modified to accommodate an emission control system of the required efficiency. For instance, it is not always possible to modify an existing facility to assure the presence of excess fuel and to remove all oxygen from the exhaust as required for efficient operation of the catalytic and selective reduction processes referred to above. Accordingly, a need exists for improved methods for reducing nitrogen oxide emissions.
It is therefore one object of this invention to provide improved processes for reducing nitrogen oxide emissions.
Another object of this invention is the provision of methods for converting nitrogen oxides to elemental nitrogen and water.
Yet another object of this invention is the provision of relatively inexpensive yet efficient methods for reducing or preventing the emission of nitrogen oxides.
Another object is the provision of improved methods for preventing the emission of nitrogen oxides from liquid systems.
Another object is the provision of improved methods for preventing the emission of nitrogen oxides from nitric acid-containing solutions and/or from solutions employed to effect the nitration of organic compounds.
Another object is the provision of improved methods for removing nitrogen oxides from vapor streams.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawing, and the appended claims.